Abstract
The Colorado River Basin Pilot Project (CRBPP), conducted in southwest Colorado during winters 1970/71–1974/75, was the largest and most expensive randomized orographic cloud-seeding experiment ever conducted in the United States. Its goal was to reproduce the 50–200% increases in snowfall reported in the Climax I experiment and in preliminary reports in two other cloud-seeding experiments. However, the CRBPP failed to demonstrate increased snowfall for four primary reasons. First, Type I statistical errors (i.e., false positives on seeded days) occurred in two of the three previous experiments, whereas the third was impacted by errors that created a seeding success. Thus, there were no successes for the CRBPP to replicate. Second, a purported close correspondence between 500-hPa and cloud-top temperatures gave the prior experiments a credible, but false, microphysical foundation. Third, a preliminary report suggesting low concentrations of ice particles in clouds with cloud-top temperatures ≥ −20°C made the clouds look ripe for seeding potential, but failed to be confirmed by later airborne and ground-based measurements of much higher concentrations. Fourth, near-surface stable air masses meant that seeding agent released from ground-based seeding equipment did not reach the clouds on many wintertime days. This review also describes the lessons learned from the CRBPP and how cloud-seeding projects have been influenced by the CRBPP: a hiatus in randomized cloud-seeding experiments in wintertime orographic clouds following the CRBPP led to the realization of the need to improve the understanding of structure and evolution of winter storms in the mountainous western United States.
© 2025 The Author(s). Published by the American Meteorological Society. This is an Author Accepted Manuscript distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) License 
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